1. Field of the Invention
The present invention relates to an electromagnetic valve used to change the pressures of hydraulic operating fluid at respective operating portions of an electronic control automatic transmission for motor vehicles, for instance.
2. Description of the Related Art
In the past, for instance, a proportional electromagnetic (or solenoid) valve has been known which includes a solenoid coil, a rod arranged on an axis of the solenoid coil for movement in an axial direction thereof, a plunger attached to an intermediate portion of the rod, a core arranged at one side of the rod so as to surround it and operable to draw or attract the plunger upon energization of the solenoid coil, a yoke arranged at the other side of the plunger so as to surround it, and a pair of bearings arranged at opposite sides of the plunger, respectively, for movably supporting the rod. One of the bearings is press-fitted into the yoke, and the other bearing is press-fitted into the core (For instance, see a first document: Japanese patent application laid-open No. 2002-188744 (FIG. 2)).
In the proportional electromagnetic (or solenoid) valve as constructed above, the pair of bearings serve to support the rod for sliding movement in the axial direction thereof, with one of the bearings being press-fitted into the yoke and the other bearing being press-fitted into the core. With such an arrangement, the central axes of the bearings do not often accurately align with each other due to variations in machined or processed parts, variations in assembly thereof, etc., as a result of which diametral unbalanced loads act on the sliding portions of the bearings and the rod. Accordingly, there arises a problem in that the sliding resistance to the rod increases and hence it is impossible to obtain a prescribed degree of opening of the proportional electromagnetic valve with respect to an amount of electric current flowing through the solenoid coil.
In addition, in order to reduce the unbalanced loads, it is possible to increase gaps or clearances between the inner peripheral surfaces of the bearings and the outer peripheral surface of the rod, as a result of which, however, the central axes of the plunger, the core and the yoke might be displaced or misaligned with respect to one another to an increased extent. Therefore, the balance between the diametral magnetic force between the plunger and the core and the diametral magnetic force between the plunger and the yoke is broken so that there arises a problem similar to the above in that diametral unbalanced loads act on the sliding portions of the bearings and the rod due to a difference between of the above-mentioned diametral magnetic forces.
Moreover, there is a further problem in that foreign matters (metal powder, dust, etc.) might bite into the sliding portions between the inner peripheral surfaces of the bearings and the outer peripheral surface of the rod, thus making the rod inoperative.